Ceramic wall flow filters are finding widening use for the removal of particulate pollutants from diesel or other combustion engine exhaust streams. A number of different approaches for manufacturing such filters from channeled honeycomb structures formed of porous ceramics are known. The most widespread approach is to position plugs of sealing material at the ends of alternate channels of such structures which can block direct fluid flow through the channels and force the fluid stream through the porous channel walls of the honeycombs before exiting the filter. The particulate filters used in diesel engine applications are formed from inorganic materials, chosen to provide excellent thermal shock resistance, low engine back-pressure, and acceptable durability in use. The most common filter compositions are based on silicon carbide, aluminum titanate and cordierite. Filter geometries are designed to minimize engine back-pressure and maximize filtration surface area per unit volume.
Diesel particulate filters can consist of a parallel array of channels with every other channel on each face sealed in a checkered pattern such that exhaust gases from the engine would have to pass through the walls of the channels in order to exit the filter. Filters of this configuration are formed by extruding a matrix that makes up the array of parallel channels and then sealing or “plugging” every other channel with a sealant in a secondary processing step. Pin holes and dimples can extend over the entire length of the plug and cause the part to be rejected by quality control procedures.
Also, during the plugging process, the preformed slug of plugging composition, which is fed into the structure part (from the ram to the interface between the slug and the structure) can run or slump in the reservoir of the piston/cylinder, resulting in difficulties in manufacturing and yields. Additionally, plugging composition formulations can have a pot life which is short due to changes in viscosity over time.
Still further, conventional plugging compositions can also contain particles that migrate into microcracks that have formed in the honeycomb structure being plugged. After drying and firing, this migration of particles into the formed microcracks can negate the CTE reducing effect of the microcracks and can result in increased coefficient of thermal expansion (CTE) for the matrix, at least at the plug surface region of the structure.